The present invention relates to a heat resistant ferritic stainless steel having improved weldability, particularly for use in articles, in which improved weldability as well as resistance to oxidation at high temperatures are required, such as a recuperator of an exhaust gas converter of automobiles and other kinds of gas combustion means.
Ferritic stainless steels, a typical one of which is the JIS SUS 430 steel corresponding to the AISI Type 430 steel, have been widely used for building materials, kitchen utensils, automobiles, etc., since such type steels are less expensive than austenitic stainless steels. The ferritic stainless steel, however, does not have good heat resistance within the temperature range of 900-1000.degree. C., and if it is subjected to a thermal cycle of heating and cooling, the spalling of scale becomes serious even at temperatures about 800-900.degree. C. Thus, the ferritic steel has been considered unsuitable for use in the exhaust gas converter of automobiles, since scale might clog such an apparatus.
Austenitic stainless steels such as JIS SUS 304 (AISI Type 304) are superior to the ferritic stainless steel in their resistance to heat. However, when the austenitic stainless steel is employed together with a carbon steel or low alloy steel, there is a danger of collapse of an assembly during service due to the large difference in their thermal expansion coefficients. Since the thermal expansion coefficient of the ferritic stainless steel is very similar to that of carbon steel, for example, in case of an automobile exhaust gas converter comprising an inner cylinder of a heat resistant steel and an outer cylinder of a carbon steel, it is desirable to make the inner cylinder out of the ferritic stainless steel. However, as already mentioned, the conventional ferritic stainless steel has inferior heat resistance. In addition, the weldability of the conventional ferritic stainless steel is unsatisfactory, so it is not suitable for the fabrication of complicated articles. Thus, it has been recognized that it is not feasible to apply the ferritic steel to such a purpose.